This invention relates to a lubricating oil filter assembly of the type used on internal combustion engines. More specifically, the invention relates to a filter assembly having a spin-on or disposable type filter cartridge adapted to be secured to a mounting adaptor on an engine having two lubricating circuits.
Even more specifically, the invention pertains to a filter assembly of the same general type as disclosed in Cudaback et al U.S. Pat. No. 5,078,877. In the Cudaback et al filter assembly, oil from the crankcase is filtered by an outer filter element and a substantial volume of the filtered oil flows to the primary lubricating circuit of the engine. A small percentage of the filtered oil is subjected to further filtration by an inner coaxial filter element and is returned directly to the crankcase. A tubular fitting extends between end caps on the two coaxial filter elements to establish communication between the inner filter element and the mounting adaptor, to separate the two flow circuits, and to maintain a fixed axial spacing between the two filter elements.
The two filter elements include filter media and Plastisol potting compound. When the media is in the form of pleated cellulose paper, both the media and the Plastisol require high temperature heating after the filter elements have been manufactured in order to cure the media. The heating process imparts moisture resistance and structural strength to the filter media and seals the elements to the end caps to prevent contamination from eluding the filtering media.
In the Cudaback et al filter assembly, the design of the end caps and the tubular fitting prevents the two filter elements from being cured individually before being assembled with one another. If the filter elements are cured after being assembled, the outer filter element acts as a heat shield with respect to the inner filter element and thus substantial time is required to complete the curing process. This problem cannot be solved by curing the inner element before it is assembled with the outer element because, under such circumstances, the inner element acts as a heat sink once it is assembled with the outer element and as the outer element is being cured. Regardless of how the curing is effected, one end cap of the outer filter element of the Cudaback et al construction cannot be installed until after the inner filter element has been inserted into the outer filter element.
In some filters, the filter media of the outer filter element is a synthetic material such as micro-glass which is sandwiched between and supported by polyester scrim. The scrim cannot withstand the high temperatures necessary to cure the less expensive cellulose paper of the inner filter element and thus, when the outer filter element uses synthetic filter media, it is essential that the inner filter element be cured prior to being assembled with the outer filter element. In the filter assembly of the Cudaback et al patent, the requirement of installing an end cap on the outer filter element after the inner filter element has been inserted into the outer element is a major drawback even if the outer element uses synthetic filter media.
One of the end caps of the outer filter element of the Cudaback et al filter assembly is a deep drawn component with punched holes defining flow passages and thus manufacture of the end cap is relatively complex and expensive. Moreover, the Cudaback et al filter suffers from the standpoint that the flow through the inner filter element may be reduced below an acceptable rate under cold start conditions or if either filter element becomes excessively clogged.